Hydrophobic Modification for the Design of High-Performance and Ultra-High Performance Cementitious Composites

This paper presents a novel method for designing high-performance and ultra-high-performance concrete (HPC/UHPC) through hydrophobic and nanomaterial modification. Three-dimensional hydrophobization of cementitious matrices is achieved by incorporating water-based emulsions of silico-organic compounds containing active hydrogen. The approach enables high-strength matrices with superior flexural performance via fiber reinforcement, further improved by uniformly distributed small air voids that promote multi-cracking and strain-hardening behavior. Incorporating aluminum oxide (Al₂O₃) nanofibers with 1% silica fume (or metakaolin) produces UHPC-grade strength and ductility while reducing chemical shrinkage and simplifying composition. Developed with Type V and I cements at W/CM < 0.3, the composites reached compressive strengths up to 195 MPa, exhibited high flowability, and showed excellent strain-hardening when reinforced with 2% synthetic macrofibers (PVA and HDPE), with HDPE yielding greater ductility. This synergy of hydrophobic modification, nanoengineered matrix, and synthetic fibers offers a cost-effective, sustainable alternative to conventional HPC/UHPC, exceeding durability and environmental performance benchmarks.